In recent years, porous inorganic compounds having uniform micropores (micropore diameter: 2 to 50 nm) has been developed. The porous inorganic compounds have larger micropores, a larger pore volume and a larger surface area than an oxide such as zeolite and the like that has been used from the past so that application of those compounds to a catalyst carrier, a separation adsorbent, a fuel cell, a sensor or the like has been studied.
As a method for preparing a porous material having these uniform micropores, a method utilizing control of the structure of an inorganic compound by the use of an organic compound has been paid attention. In particular, it has been known that an oxide having uniform micropores, that is formed by utilizing a cooperative organization (self-organization) due to an interaction between an organic compound and an inorganic compound, has a larger pore volume and a larger surface area than the conventional zeolite and the like.
There has been disclosed a method for preparing a porous material having uniform micropores utilizing a cooperative self-organization due to an interaction between an organic compound and an inorganic compound, for example, in WO 91/11390. Specifically, a method comprising subjecting a precursor solution comprising a silica gel, a surfactant and the like to a hydrothermal synthesis in a heat-resistant closed vessel to prepare porous silica has been disclosed.
Furthermore, in Bull. Chem. Soc. Jp., Vol. 63, p. 988 (1990), a method comprising subjecting kanemite that is a kind of a layered silicate and a surfactant to ion exchange to prepare such a porous material has been disclosed.
In order to use a porous material having uniform micropores for an optically functional material, an electronically functional material or the like, there has been recently reported a method for preparing porous silica in the form of a film. For example, in Nature, Vol. 379, p. 703 (1996), J. Am. Chem. Soc., Vol. 121, p. 7618 (1999) or the like is described a method for forming a film by immersing a substrate in a sol solution comprising a condensation product of alkoxysilanes and a surfactant, and depositing porous silica having uniform micropores on the substrate surface.
Furthermore, in Chem. Commun., p. 1149 (1996), Supramolecular Science, Vol. 5, 247 (1998), Adv. Mater., Vol. 10, p. 1280 (1998), Nature, Vol. 389, p. 364 (1997), Nature, Vol. 398, p. 223 (1999), or the like is described a method for forming a film on a substrate by coating the substrate with a coating liquid in which a condensation product of alkoxysilanes and a surfactant are mixed in an organic solvent, and subsequently evaporating the organic solvent.
When the porous film having uniform micropores is used as a low permittivity insulating film of an integrated circuit, it is required that a metal ion such as an alkali metal including sodium or potassium, or the like is strictly removed. A positively charged ion, when exposed to an electric field, easily moves, because of its drift from a positively biased film to a negatively biased film, causing the shift in capacitance or voltage. Accordingly, in order to obtain a porous film which is preferably used as an insulating film of an optically functional material and an electronically functional material, it is required that a metal, i.e., an impurity is not contained in a coating liquid used for preparing a porous film. Namely, in order to prepare such a coating liquid, it needs to remove a metal, i.e., an impurity, from a surfactant, an organic solvent or the like used as a raw material.
However, when a metal ion such as an alkali metal ion or the like is removed from the coating liquid, zeta potential of a silica oligomer in the coating liquid is changed, and the mesophase of the silica oligomer and the surfactant becomes unstable. As a result, there has been a problem such that the regularity in micropores is deteriorated due to the time from the preparation of a coating liquid until coating. Consequently, a coating liquid such that the regularity in micropores or the permittivity of the obtained porous silica film becomes uniform regardless of the shelf life, even if a metal ion is removed, has been in demand.
In order to solve the problem, for example, in JP2002-26003A is described that, even if a metal ion is removed, a porous silica film having uniform micropores is obtained regardless of the shelf life when a coating liquid containing a tetraalkylammonium salt, a tetraorgano ammonium salt or an organoamine in an acidic medium is used.
However, there are problems such that the tetralkylammonium salt, the tetraorgano ammonium salt and organoamine are expensive, and organoamine is very toxic.
Furthermore, in Microporous and Mespporous materials, Vols. 35-36, p. 545 (2000), organoamine is described, which penetrates into the inside of the surfactant, affecting the regularity in micropores or a pore size. For this reason, when an amine described in JP2002-26003A is used as a coating liquid, physical properties such as the mechanical strength or the like of the obtained porous silica can be possibly deteriorated.
Under the circumstances, a coating liquid for forming porous silica having the following properties has been in demand,
porous silica which does not cause the shift in capacitance or voltage is obtained even when it is exposed to an electric field;
the regularity in micropores, the permittivity or the mechanical strength of the obtained porous silica is not changed regardless of its self-life time;
the cost is cheap; and
the safety is high.